This invention relates to photographic systems which employ an infrared ranging system and, more particularly, it concerns such systems in which infrared light emitted from a camera mounted flash source is used for rangefinding during an exposure cycle irrespective of whether exposure is effected by flash or ambient light.
Automatic ranging cameras are known to use a preliminary flash increment of illumination or "wink" to determine the camera-to-subject distance by measuring the subject reflected component of the wink and relating the intensity of that component to distance for lens focusing adjustment. Such cameras incorporate a photocell to detect the amount of wink radiation reflected by the subject and appropriate electronic circuitry to compare the reflected radiation value either to an initial radiation value or with the intensity value of the wink in order to compute the camera-to-subject distance. While such wink light systems are effective as rangefinders, when the source of the wink is visible light, the subject being photographed may be disturbed, or the rangefinding accuracy hampered by differing values of scene reflectivity. Accordingly, photographic systems employing an infrared ranging wink have been disclosed to avoid disturbance to the subject and to reduce the adverse influence of variations in scene reflectivity.
The use of a preliminary infrared flash or wink in advance of a main flash of visible light to determine the camera-to-subject distance by detecting reflected infrared radiation is known in photographic systems which employ two separate flash elements; one flash element producing the infrared wink and the other flash element producing the visible light exposure flash. One example of such a ranging system is described in U.S. Pat. No. 4,457,611 entitled "PHOTOGRAPHIC SYSTEM FOR USE IN FLASH PHOTOGRAPHY WITH A CAMERA", issued on July 3, 1984 to T. Ishida et al. The preliminary infrared wink is produced by a preliminary flash tube behind a filter which cuts off visible light and allows infrared light to pass. For exposure, a main flash of visible light is produced by a separate flash tube. In addition to the requirement for two separate flash tubes, this type of system requires a complicated dual control system, takes up camera space, increases the consumption of energy, and increases the cost of manufacture.
Another known photographic ranging system employs a single flash tube which emits both a preliminary visible light flash or wink and a main visible light flash. The reflected light energy sensor in this system includes a filter which blocks out all the visible light and allows only infrared light to pass through the sensor element. Examples of such ranging systems incorporated in automatic ranging cameras are disclosed in U.S. Pat. No. 4,357,083 entitled "METHOD AND APPARATUS USING WEIGHTED RANGE SIGNAL FOR CONTROLLING PHOTOGRAPHIC FUNCTIONS", issued to B. K. Johnson et al on Nov. 2, 1982; U.S. Pat. No. 4,549,801 entitled "AUTOMATIC FOCUSING CAMERA WITH AUTOMATIC APERTURE SETTING", issued to A. J. Winter on Oct. 29, 1985 and British Published Patent Application No. 2,110,828 entitled "AUTOMATIC FOCUSING" issued to A. J. Winter on June 22, 1983. All three of these systems suffer the drawback that the preliminary flash includes visible light which may cause the subject being photographed to react to the preliminary flash by blinking, for example, and result in a picture with such undesirable characteristics as partially or fully closed eyes in the face of a human subject.
As disclosed in U.S. Pat. No. 4,415,245 entitled "MOVABLE INFRARED FILTER FOR AN AUTO RANGING CAMERA", issued to D. M. Harvey on Nov. 15, 1983, a visible light blocking filter is placed in front of a single camera mounted flash tube for the ambient mode operation when flash exposure is not provided. In this mode, the visible light blocking filter is placed in front of the flash tube during a pre-exposure flash or wink for ranging purposes so that the automatic focusing system is operated without disturbing the subject or surroundings to be photographed during the ranging operation. However, in the flash mode where an exposure flash is desired, this system does not utilize the visible light blocking filter, and thus requires the emission of a rangefinding light flash or wink of visible light when a main light flash is desired for exposure.
The pre-exposure flash or wink ranging photographic systems of the prior art, therefore, suffer the drawbacks either of being overly complicated by the requirement of two separate flash tubes or of being disturbing to the subject or surroundings to be photographed by use of visible light for the preliminary flash or wink.